The invention relates to making composite material parts, and more particularly to making reinforcing fiber structures for such parts.
The field of application of the invention is making structural parts of composite material having fiber reinforcement and a resin matrix. Such parts are used in very many fields, and in particular in the field of aviation. One particular example is making blades for turbomachines, in particular fan blades. The reinforcing fiber structure is densified by a resin matrix in any known manner, for example by resin transfer molding (RTM).
Another field of application for the invention is making parts out of thermostructural composite material, i.e. a material having mechanical properties that makes it suitable for constituting structural elements and also having the ability to conserve these properties at high temperatures. Thermostructural composite materials are typically carbon/carbon (C/C) composite materials having fiber reinforcement of carbon densified by a carbon matrix, and ceramic matrix composite (CMC) materials having refractory fiber reinforcement (of carbon or ceramic) densified by a ceramic matrix. Thermostructural composite material parts are used in particular in the fields of aviation and space flight. The reinforcing fiber structure can be densified by the material that constitutes the matrix by performing chemical vapor infiltration (CVI) or by using a liquid technique, as is well known. Liquid densification consists in impregnating the fiber structure with a liquid composition containing a precursor for the material that is to constitute the matrix, typically a resin, with the precursor being transformed by heat treatment.
For composite material parts that present a certain amount of thickness, it is known to make the reinforcing fiber structure as a plurality of superposed layers that are bonded to one another so as to avoid the layers separating, in particular it is known to make a reinforcing structure by three-dimensional weaving.
Thus, document EP 1 526 285 proposes making a reinforcing structure or preform for a turbomachine blade, in particular a fan blade, by interlock type three-dimensional weaving with the weave varying in the longitudinal direction between the root and the airfoil of a blade.
Various aspects need to be taken into consideration when making reinforcing fiber structures for composite material parts.
Thus, when composite material parts need to present a smooth surface state, it is desirable to avoid the reinforcing fiber structure presenting large surface irregularities that require additional operations in order to eliminate or avoid such irregularities, such as surface trimming, e.g. after a first stage of densification or consolidation stage, or adding a two-dimensional ply on the surface, e.g. a ply of woven fabric, before the end of densification.
In addition, for composite material parts having a resin matrix, the presence of significant surface irregularities leads to the formation of localized clumping of the resin, thereby harming the mechanical properties of such parts.
Furthermore, for a composite material part that is obtained by densifying a fiber reinforcing structure by CVI, it is desirable to leave easy access for the infiltration to reach the core of the fiber structure, and to ensure that the core presents porosity that is relatively uniform so that densification presents as little non-uniformity as possible. When access to the pores within the fiber structure is difficult, or when there are pores presenting sizes that are very different, with the smaller pores becoming closed up more quickly, then a steep densification gradient is inevitable, and that affects the properties of the composite material.
Finally, with composite material parts presenting thin portions, such as, for example, the trailing edges of turbomachine blades, it is desirable to conserve a three-dimensional structure that extends into said thin portions, while preserving structural continuity with the thicker portions, so as to impart the expected mechanical properties to the parts.